remove github_token parameter and point to help docs instead

I want the action to be very easy to understand and use. So I removed the mention of the optional github_token parameter  and instead pointed people to a wiki help page if they want to provide their token explicitly.

README.md

@@ -7,10 +7,6 @@ A Github action that determines if a tag exists
 
 **Required** The tag to search for.
 
-### `github_token`
-
-GitHub token. default: `${{github.token}}`
-
 ## Outputs
 
 ### `exists`
@@ -27,3 +23,5 @@ a string value of 'true' or 'false'
 
 - run: echo ${{ steps.checkTag.outputs.exists }}
 ```
+
+This action uses the `${{github.token}}` secret to automatically inject your access token. If you'd like to provide your own token instead check out [this help article](https://github.com/mukunku/tag-exists-action/wiki/Setting-the-GITHUB_TOKEN-explicitly)